According to David Bus:

* Physical reasons such as to reduce stress ("It seemed like good exercise"), feel pleasure ("It's exciting"), improve or expand experiences ("I was curious about sex"), and the physical desirability of their partner ("The person was a good dancer").

* Goal-based reasons, including utilitarian or practical considerations ("I wanted to have a baby"), social status ("I wanted to be popular") and revenge ("I wanted to give someone else a sexually transmitted disease").

* Emotional reasons such as love and commitment ("I wanted to feel connected") and expression ("I wanted to say 'thank you'").

* Insecurity-based reasons, including self-esteem ("I wanted the attention"), a feeling of duty or pressure ("My partner kept insisting") and to guard a mate ("I wanted to keep my partner from straying").

Whew... that's cleared up now. But why do you have sex?!!?